The present invention relates to an image forming apparatus, for example, a copying machine, a printer, a facsimile machine, and the like, for electrostatically forming an image on a recording medium.
There have been known such recording apparatuses as copying machines, laser beam printers or the like, that form full-color images by repeating for yellow, magenta, cyan, and black toners, a process in which a toner image is formed on a photosensitive drum, and the toner image is transferred from the photosensitive drum onto a recording medium borne on the recording medium bearing portion (peripheral surface) of a transfer drum.
Briefly describing such an image forming apparatus, first, a recording medium is conveyed to a transfer drum with a predetermined timing, and adhesion current is flowed to the back side of the transfer sheet from an adhesion brush (at this moment, an electrically conductive roller is in contact with the recording medium), causing the recording medium to be electrostatically adhered to the transfer sheet. Then, the yellow color toner image which was formed on the photosensitive drum is transferred onto the recording medium having been adhered to the transfer sheet. This process is also carried out for magenta, cyan, and black toners. As a result, a full-color image is formed on the recording medium. In this transfer process, a voltage with a predetermined level is applied to the inward side of the transfer sheet by a transfer charging device constituted of a transfer brush or the like. The recording medium on which a full-color image has been formed is separated from the transfer sheet, and the toner image is fixed to the recording medium by a fixing device. Thereafter, the recording medium is discharged from the image forming apparatus. The transfer sheet from which the recording medium has been separated is cleared of electrical charge by an inside charge removing device and an outside charger removing device, which are on the inward and outward sides, respectively, of the transfer sheet, and then, its surface is cleaned by a cleaning apparatus.
In the case of the aforementioned full-color image forming apparatus, the electrical potential on the surface of the transfer sheet fluctuates as follows. That is, first, the surface of the transfer sheet is initialized by the inside and outside charge removing devices. It is assumed that the voltage level of the transfer sheet after the charge removal is V0. Thereafter, the voltage level on the back surface of the transfer sheet increases by Vq as an adhesion voltage is applied to the back surface of the transfer sheet in order to bear and hold a sheet of recording medium on the transfer sheet. Also in the transfer process, the back surface potential of the transfer sheet fluctuates by a voltage level of Vt as transfer voltage is applied to the transfer sheet from the transfer brush.
In the case of the full-color image forming apparatus (full-color copying machine), this transfer process is carried out four times per full-color image, and then, the recording medium is separated from the transfer sheet. Assuming that the voltage level of the transfer voltage is constant, the voltage level on the back surface of the transfer sheet increases by V1=V0+Vq+4xc3x97Vt, through a total of four transfer processes.
As is commonly known, edges of a sheet as recording medium have burrs. If these burrs at the edges of a recording sheet face the transfer drum, and the transfer voltage is applied, starting from the leading edge of the recording sheet, such force that causes the recording sheet and the photosensitive drum to be attracted to each other is generated. As a result, the force which causes the leading edge of the recording sheet to be attracted to the transfer drum is reduced, allowing the leading edge of the recording sheet to be lifted from the transfer drum. This results in the production of an image of low quality traceable to transfer failure. In an extreme case, the recording sheet is wrapped around the photosensitive drum, making recording sheet conveyance impossible.
Thus, in the case of this image forming apparatus, control is executed so that transfer current does not flow at the leading edge of a recording sheet. More specifically, where the leading edge of a recording sheet is on the transfer sheet, the potential level of the transfer brush is made to be the same as that of the transfer sheet so that current does not flow from the transfer brush to the transfer sheet (and recording sheet).
In the region in which control is carried out to prevent transfer current from flowing, the potential level of the transfer sheet is kept at the initial state; it remains at V0+Vq until the end of the formation of each full-color image. Therefore, in the case of an image forming apparatus such as this image forming apparatus, the level of transfer voltage must be increased from V0+Vq to Vlast (Vlast greater than V1) during the period from when the leading edge of a recording sheet, that is, the leading edge of the leading margin portion of a recording sheet, makes contact with the transfer sheet, to when the trailing edge of the leading margin portion of the recording makes contact with the transfer sheet. This fluctuation in the transfer voltage is shown in FIG. 8.
When the capacity of a transfer sheet is Cpt; the capacity of a recording medium is Cpp; the amounts of electrical charge necessary for the adhesion and the transfer are Qq and Qt, respectively (these values are dependent upon the characteristic of the photosensitive drum or the amount of toner charge, and in a stable environment they are not dependent upon the characteristic of the transfer sheet or the recording medium; they remain stable), Vq=Qqxc3x97(1/Cpt+1/Cpp), and Vt=Qtxc3x97(1/Cpt+1/Cpp).
In the past, 150 xcexcm thick polycarbonate has been used as the material for the transfer sheet. However, the dielectric constant of the polycarbonate is approximately 3, relatively low compared to that of PVDF (polyvinylidene), which is 10. Further, the capacity Cpt of the transfer sheet is proportional to the dielectric constant of the transfer sheet. Therefore, the usage of the polycarbonate as the material for the transfer sheet made the transfer sheet capacity Cpt small. As a result, the amount of the electrical charge necessary for the transfer process makes the values of the Vq and Vt rather large.
Further, in an environment with low humidity, or as the thickness of a recording medium is increased, the dielectric constant of the recording medium becomes extremely small, which also increases the values of the Vq and Vt. In reality, the average dielectric constant of a sheet of recording paper under the low humidity condition is 2-4. For example, if a recording sheet is approximately 100 xcexcm in thickness, and 3 in dielectric constant, the amount by which the potential level of a transfer sheet or a recording sheet increases is approximately 4-8 kV. It is extremely difficult to increase the potential level of the transfer sheet or the recording sheet by this much amount within a short period equivalent to the very narrow width, in terms of the sheet conveyance direction, of the leading margin portion of the recording sheet (if the width of the margin is 8 mm, and the process speed is 200 mm/sec, the potential level must be increased by 4-8 kV in 40 milliseconds). As a result, the problem frequently occurs that a copy lacking the portion of the image at the leading end is produced.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus, method and a control system therefor wherein a defect adjacent a leading edge of a recording material is suppressed or removed.
It is another object of the present invention to provide an image forming apparatus, method and a control system therefor wherein a clear image can be formed from the leading edge of the recording material.
According to an aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member for bearing an image; a movable recording material carrying member for electrostatically carrying the recording material; transfer means for electrostatically transferring the image from said image bearing member onto the recording material carried on said recording material carrying member at a transfer position, said transfer means being located at a position across from a side of said recording material carrying member carrying the recording material; and control means for controlling timing of start of change of a current flowing through said transfer means in accordance with a kind of the recording material so that the current reaches a level effective to transfer the image from said image bearing member onto the recording material carried on said recording material carrying member within a period in which a non-image formation area of the recording material at its leading edge is passing the transfer position.